Integrated circuit cards (or “PC cards”) are often used in mobile communications and computing. In one particular application, the PC card comprises a wireless modem that plugs into a PCMCIA slot of a laptop computer or personal digital assistant (PDA). PCMCIA is an acronym for “Personal Computer Memory Card International Association”, which sets forth standards for PC cards. Typically, the host device (i.e. laptop or PDA) is coupled to a direct current to direct current (DC/DC) converter, which converts the host power supply voltage to a voltage source that powers the PC card.
For efficient use of the converted power by the wireless modem, the output impedance of the PC card must be well matched to the input impedance of the modem antenna. An impedance mismatch can occur if the antenna is not oriented for proper transmission, is broken or is in contact with an object that inhibits its ability to radiate radio frequency (RF) power. A consequence of the mismatch is the transmission of a weak RF signal and the possibility of the following two scenarios occurring. First, the power control loop of the power amplifier (PA) in the wireless modem will act to increase the PA output power to compensate PA output loss due to antenna mismatch. Second, when the base station of the wireless network receives the weak RF signal, the base station will send a request for the modem to increase its output power, not knowing that the weak signal is attributable to the antenna mismatch. Both of these scenarios may result in the PA of the wireless modem drawing excessive current from the host power supply, i.e., more current than the supply is designed to properly supply. This phenomenon is often referred to as “current overdraw.” Current overdraw is undesirable since it can damage the power supply, cause the host device to reset, and/or cause the modem to overheat.
A solution to preventing current overdraw would be to use a fixed hardware limiter to limit the power amplifier DC power rail. Unfortunately, this solution has two problems. First, conventional hardware current limiters only guarantee about a 20% threshold accuracy. This degree of uncertainty in accuracy may be too large to manage and prevent current overdraw by the power amplifier. Second, using a fixed hardware limit may result in the sending of a “false alarm” of a current overdraw condition to a host that may, in fact, have the ability to deliver a particular current draw demand (i.e. a “strong” power supply). Conversely, using a fixed hard limit may result in the sending of no warning at all of a current overdraw condition to a host that may not have the ability to deliver a particular current draw demand (i.e. a “weak” power supply). These problems would be compounded in multiple-time-slotted systems, in which current demands vary depending on the number of transmission slots allocated per transmission burst.